The present invention is generally directed to an electronic process instrument for sensing a process variable in an industrial process. More particularly, the present invention is directed to a combined sensor, switch, gauge, and transmitter for displaying (with the gauge) and transmitting (with the transmitter) a value corresponding to a condition or variable that is sensed by the sensor, and for switching the switch when a predetermined threshold value of the condition is sensed.
In one embodiment of the invention, an electronic process instrument has a sensor, for sensing a condition in an environment, and a transmitter for transmitting information indicative of that which is sensed by the sensor. In particular, the sensor is powered by a dc power supply via is a two-wire loop. In accordance with an aspect of the invention, information indicative of that which is sensed by the sensor is transmitted via the two-wire loop by modulating the amplitude of an electrical current flowing in the loop corresponding to that which is sensed by the sensor. Preferably, the electrical current flowing in the two-wire loop, and providing an indication of that which is sensed by the sensor, is between 4 and 20 milliamps when the sensor is operating. For example, when the sensor is a pressure sensor for sensing the pressure of a fluid flowing in a line, the electrical current is controlled so as to correspond to the pressure of the fluid, as sensed by the sensor.
More specifically, the present invention has a controller, such as a microprocessor. Connected to the microprocessor is the sensor for sensing a condition in an environment in which the switch is placed. As alluded to, in a preferred embodiment, the sensor is a pressure sensor for sensing the pressure of a fluid flowing in a line, although it should be understood that the sensor may be a temperature or other type of sensor. Also connected to the processor is a gauge for displaying information indicative of that which is sensed by the sensor.
The components of the invention and, particularly, the sensor, gauge, and controller, are powered by a dc power supply via a common two-wire loop. The controller receives data, from the sensor, indicative of the condition being sensed. In accordance with the invention, the controller utilizes the two-wire power supply loop to transmit information indicative of the received data. In particular, the controller controls the amplitude of the electrical current flowing on the two-wire power supply loop so that the current corresponds to the data received from the sensor. Preferably, the electrical current flowing in the two-wire loop is adjusted within the range of 4 and 20 milliamps to indicate the level of the corresponding variable sensed by the sensor.
By providing an instrument according to the present invention, numerous advantages are achieved. Among these advantages, the instrument significantly reduces the response time required to sense the process variable and transmit the output through the two wire power supply loop. Preferably, the response time is less than 30 milliseconds. Transmission through the two wire loop with built-in diagnostics is accurate and fail safe and results in longer, improved mean time before failure of the device.
Another advantage of the instrument of the present invention is that the switch or switches, gauge and transmitter are powered by a single source and are independently scalable. Thus, for instance, since the components are independent, diagnostics may be run and/or the transmission output may be programmed without affecting the gauge output or switch operation. Moreover, the switch or switches may be tested without affecting the transmission output, and likewise, testing of the transmitter may be performed without affecting the switch(es). Since the components are scalable, the process variables may be displayed or transmitted when the process variable is at a level outside the normal operating range of the device.
In addition to accomplishing these and other advantages, the device is universal and may be used in conjunction with sensors for sensing a number of process variables such as pressure, temperature, volumetric flow, level, or differential pressure or temperature. Also, the components of the device may be located in a single, compact housing placed in communication with the line or system.